The preferred embodiment concerns a continuous intermediate image carrier for an electrophotographic printer or copier that serves for acquisition, transport and/or delivery of a toner image in the electrophotographic printer or copier. A plurality of known electrophotographic printers, in particular color printers, comprise an intermediate carrier medium, advantageously a transfer belt. Individual color separations generated on a photoconductor with the aid of an electrophotographic method are successively printed in register one atop another from this photoconductor onto an intermediate carrier medium and are thereby collected on the intermediate carrier medium. The color separations printed over one another are subsequently transferred from the intermediate carrier medium onto a carrier material to be printed. Such known intermediate carrier media are typically comprised of synthetics (in particular elastomers) with a constant electrical conductivity. These known printers are typically single sheet printers with process speeds of <200 DIN A4 pages per minute. Such known intermediate carrier media are not suitable for qualitatively high-grade print results given process speeds of >200 pages A4 per minute.
The previously-known intermediate carrier media can essentially be associated into two groups. The intermediate carrier media of the first group are high-ohmic, whereby small transfer printing currents are required. Given small transfer printing currents, high-voltage power supplies with low efficiency can be cost-effectively used. Given these high-ohmic intermediate carrier media, the toner transfer onto the intermediate carrier medium and from the intermediate carrier medium also occurs with a relatively high efficiency. However, given the use of high-ohmic intermediate carrier media it is disadvantageous that it leads to what is known as a spraying of small characters even at relatively low process speeds, whereby the print quality is reduced. Given increasing process speeds it also leads to an electrostatic charging of the surface of the intermediate carrier medium.
Such an electrostatic charging leads to a destruction of the print image transferred onto the intermediate carrier medium due to sporadic, uncontrollable discharges. Given these discharges what are known as Lichtenberg figures are generated via which the print image located on the intermediate carrier medium is at least partially destroyed. The specific volume resistivity determined (with the aid of a measurement arrangement described in connection with FIGS. 4 through 8) at 10 V measurement voltage is greater than or equal to 1012 Ωcm given intermediate carrier media of the first group.
Relative to the intermediate carrier media of the first group, the intermediate carrier media of the second group are relatively low-ohmic. The specific volume resistivity determined (with the aid of a measurement arrangement described in connection with FIGS. 4 through 8) at 10 V measurement voltage is less than or equal to 1010 Ωcm given intermediate carrier media of the first group. Given these intermediate carrier media the sporadic, uncontrollable discharges are in fact prevented; however, the transfer of the toner images onto the intermediate carrier medium or from the intermediate carrier medium occurs with a relatively poor efficiency. Given low process speeds, a still-sufficient transfer of the toner images occurs via a relatively long residence time in the transfer printing region. In printers with intermediate carrier media of the second group, it is also known to use additional wax blades and Teflon rods that contact the surface of the intermediate carrier medium in order to reduce the surface energy of the intermediate carrier medium. The adhesion forces of the toner particles on the intermediate carrier medium should thereby be reduced and the toner transfer in the transfer printing regions should be made easier. However, in high-capacity printers with a print capacity of >200 sheets A4 per minute the transfer printing efficiency is significantly reduced due to the reduction of the residence time of the toner in the transfer printing regions at higher process speeds. The mentioned techniques for influencing the surface energy of the intermediate carrier medium then no longer lead to acceptable print results since the service lives of the intermediate carrier media are reduced via these techniques. Given a double-sided transfer printing of toner images onto carrier materials to be printed, further problems occur when the carrier material to be printed has a smaller width than the width of the intermediate carrier medium. This arrangement leads to a charge carrier exchange between the intermediate carrier media directly contacting in the regions adjacent to the carrier material when a first toner image on a first intermediate carrier medium is transfer-printed onto the front side of the carrier material and a second toner image is transfer-printed from a second intermediate carrier medium onto the back side of the carrier material in a common transfer printing region and the surfaces of the intermediate carrier media contact at least in one region adjacent to the carrier material. The intermediate carrier media adjacent to the carrier material are in direct contact, whereby an equalization current flows past laterally to the print substance. Due to this equalization current and the exchange of the charge carriers thereby affected given contacting of the surfaces of the intermediate carrier media, an interruption of the electrical field in the transfer printing region occurs as a result of the relatively good electrical conductivity of the low-ohmic intermediate carrier media.
Known intermediate carrier media are characterized by parameters specified in standards (such as, for example, ASTM D257 or IEC 60093), in particular characterized by the specific volume resistivity and the specific surface resistance. It is thereby assumed that the electrical properties of the intermediate carrier material are homogeneous and exhibit no direction-dependent properties.
A transfer belt that is comprised of at least two layers is known from the document JP-A-2000 315 020, whereby the upper layer has a higher resistance value than the other layers.
A transfer belt on whose top side are arranged two oppositely-situated layers is known from the document JP-A-11 352 785, whereby the volume resistivity of the outer layer is smaller than the volume resistivity of the underlying layer. The outer layer serves as a discharge layer. A transfer roller that comprises a plurality of layers arranged atop one another is known from the document JP-A-11 073 036, whereby at least one layer comprises a conductive powder (such as carbon or conductive metal oxide) that is arranged distributed in a polymer material.
An arrangement is known from the document JP-A-2001 034 074 in which the resistance of a continuous belt is determined in the thickness direction with the aid of two oppositely-situated electrodes.